This invention relates to low-distortion amplifiers, and particularly to feedforward amplifiers.
Feedforward is a well-known technique for reducing the effects of distortion and noise products generated by an amplifier. A sample of the amplifier's output signal is compared with a sample of its input signal to produce an error signal proportional to the internally generated distortion and noise products, and the error signal is amplified and combined with the amplifier's output signal in opposite phase to substantially cancel the distortion and noise products.
Feedforward is particularly useful in signal transmission systems employing coaxial lines, where amplifiers are often connected in cascade for periodic signal boosting to overcome line attenuation. Cascading of amplifiers is routine in CATV systems, for example, where television and other signals are transmitted from a central transmitting station, known as the head end, via a coaxial line network having one or more trunk lines and a number of feeder lines connected to each trunk line. Trunk amplifiers are spaced at appropriate intervals along each trunk line to amplify signals being transmitted through the system, and bridging amplifiers are connected to trunk amplifiers to provide multiple high-level outputs for driving feeder lines. Subscriber taps are coupled to the feeder lines, which typically also include line extender amplifiers which boost the transmitted signals and thereby extend the feeder lines. Many systems also include a return trunk for two-way transmission. A feedforward topology reduces distortion products generated in the process of signal amplification and thereby allows operation at higher signal levels at the output of any one amplifier station with an acceptable level of distortion, thus facilitating bandwidth expansion, i.e., carriage of more channels without incurring significant degradation in distortion performance. The higher output levels are required to overcome the increased cable attenuation at higher frequencies.
A 20 dB reduction in third order distortions such as composite triple beat and crossmodulation effects can be achieved with feedforward topologies, but with power consumption three times that of a hybrid amplifier, the conventional gain block in broadband CATV distribution systems prior to the use of feedforward. The localized threefold increase in power associated with the feedforward device requires a copper heat spreader to distribute the heat throughout the station. An adequate heat spreader occupies a substantial amount of space in an amplifier station and, due to labor-intensive machining, is an expensive item. Without considering heat spreader costs, a feedforward device already costs approximately ten times as much as a conventional hybrid module. Because of these and other limitations, the heat spreader concept seriously impairs upgradability, via field retrofits, to existing stations. In some instances, field upgrading of trunk stations to feedforward requires removal of all modules, including, as will be described later, forward, reverse and bridger amplifiers and AGC, so that the heat spreader may be installed in the station. In addition to the inconvenience to the technician in installing the new equipment, there would be an undesired interruption in service along the return segment of the trunk. Some systems employ telemetry and security systems along the return trunk, and a breach of security is unacceptable. Moreover, although a heat spreader allows feedforward in a trunk amplifier module, it is ineffective at heat dispersion when feedforward is also used in the bridger module, where it is required for system upgrades in bandwidth.
Another problem with current feedforward amplifiers involves high-gain hybrids, that is, hybrids having gain of 34-38 dB, which are commonly used as building blocks for such amplifiers, as will be described later in more detail. High-gain hybrids have frequency response problems which hybrid manufacturers have not adequately resolved for devices operating from a negative power supply. Since adequate negative-supply feedforward devices are lacking, existing product lines using a negative power supply cannot be upgraded to feedforward topology without installation of a dual-polarity power supply in the station to operate positive-supply feedforward devices along with the negative-supply conventional devices utilized in the other amplifier modules. This causes a serious impact in station space availability and increases cost.